Pressure-sensitive adhesion properties or intrinsic adhesion properties are not typical of polyurethanes. Although polyurethanes occupy position five in the ranking list of plastics in terms of the quantities produced, PSAs of this material play only a very minor role economically.
Nevertheless, polyurethane PSAs have been known for a long time and are described diversely. It has likewise long been known that the effect of pressure-sensitive adhesiveness can be obtained by adding tackifier resins and/or plasticizers to the polyurethane base polymer.
This method is described in, for example, U.S. Pat. No. 3,437,622 A (Dahl et al., Continental Tapes), U.S. Pat. No. 3,718,712 A (Tushaus et al., 3M), U.S. Pat. No. 4,087,392 A (Hartmann et al., BASF), DE 19 04 102 A (Hagenweiler, BASF), and JP 2000 256 639 (Toyo).
The pressure-sensitive adhesives described to date in this way have the disadvantage, on the one hand, of exhibiting strong peel increase on the majority of substrates, especially on paper or paperboard, when bonded for a prolonged period, and hence of being impossible to detach again without damage, and, on the other hand, of yielding even under slight shearing loads, with the consequence of rapid, cohesive failure of the bond.
The term “peel increase” is understood by a person skilled in the art to describe the increase in bond strength of the adhesive bond during storage.
It is also possible for the tackifier resin to migrate into the substrate, where it leaves greasy-looking spots. In addition, the resin-containing pressure-sensitive adhesives described to date that are based on polyurethane are not transparent.
The highly disruptive phenomenon of the PSA, following its redetachment from paper, paperboard, wallpaper or similar materials, leaving behind “grease spots” is observed not least for those polyurethane-based PSAs where the pressure-sensitive adhesiveness is obtained by undercrosslinking: i.e. an amount of isocyanate groups in deficit to the isocyanate-reactive groups, such as hydroxyl or amino groups, for example.
PSAs designed on the principle of undercrosslinking are described in, for example, U.S. Pat. No. 5,157,101 A (Orr, Norwood), DE 24 35 218 A (Adsley et al., Adhesive Tapes), JP 59 227 922 A1 (Sanyo), U.S. Pat. No. 3,930,102 A (Szonn et al., Beiersdorf), U.S. Pat. No. 5,714,543 A (Kydonieus et al., Bristol Myers Squibb), EP 0 597 636 A1 (Kydonieus et al., Squibb) and U.S. Pat. No. 5,591,820 A (Kydonieus et al., Squibb).
Polyurethane PSAs containing monools fall into a very similar category with analogous weaknesses. Polyurethanes of this kind are likewise undercrosslinked and therefore contain relatively large fractions of migratable polyurethane units of low molecular weight.
Polyurethane PSAs on this basis are known from, for example, EP 0 882 749 A1 (Ikeda et al., Nitto), U.S. Pat. No. 5,227,409 A (Mobley et al., Dow) and U.S. Pat. No. 5,102,714 A (Mobley et al., Dow).
Another type of polyurethane PSAs uses polyol components which carry hydroxyl groups and contain double bonds. Polyurethane PSAs on this basis are set out in, for example, JP 02 003 476 A1 (Tsubota et al., Shinko), WO 98/30648 A1 (Gerard et al., Shell), JP 59 230 076 A1 (Sekisui), JP 2001 146 577 A1 (Toyo), U.S. Pat. No. 3,879,248 A (Kest), U.S. Pat. No. 3,743,616 A (Kest), U.S. Pat. No. 3,743,617 A (Kest), U.S. Pat. No. 5,486,570 A (St. Clair, Shell) and U.S. Pat. No. 3,515,773 A (Dahl et al., Continental Tapes). A drawback is the oxidative sensitivity of these PSAs, caused by the double bonds in the polymer main chain.
After a certain time this leads to filming or to “blunting” of the pressure-sensitively adhesive surface. In addition, the majority of PSAs of this type further contain resins, with the disadvantages already described earlier on above.
A special polyurethane PSA containing carbon-carbon double bonds and based on castor oil, a natural product, is described in U.S. Pat. No. 3,246,049 A (Webber, Norton). Here again, the oxidative sensitivity is to be regarded as a weakness.
EP 0 979 835 A (Questel et al., Elf Atochem) proposes hydroxyl-terminated polyalkylenes as a polyol component, which would solve the problem of oxidative activity. The compositions, however, are moisture-curing, consequently attain a high ultimate cohesive strength and cannot be used more than once, so that they are unsuitable for reversible adhesive bonding paper. They, further contain tackifier resins and plasticizers, whose disadvantages have already been described earlier on above.
Moisture-curing polyurethane PSAs are also described in, for example, U.S. Pat. No. 4,661,542 A (USM), JP 63 189 486 A1 (Sanyo) and EP 0 196 749 A1 (von Voithenberg et al., Emhart).
A polyurethane PSA based on hydrogenated polybutadienes is described in JP 01 156 386 A1 (Uehara et al., Hitachi). A drawback there is the need for electron beam crosslinking, which involves a considerable level of technical complexity.
A polyurethane PSA likewise requiring electron beam curing is known from JP 63 260 977 A1 (Uehara et al., Hitachi). It uses polyethers as polyol component.
Certain publications describe polyurethane-including blends or polyurethane copolymers having pressure-sensitive adhesive properties. Examples include U.S. Pat. No. 5,910,536 A (Kydonieus et al., Bristol Myers Squibb), U.S. Pat. No. 5,714,543 A (Shah et al., Bristol Myers Squibb) and U.S. Pat. No. 4,626,475 A (Barnett et al., Ashland Oil). These PSAs generally feature a heightened tack and are therefore difficult to remove from paper and other sensitive substrates without damaging them.
Polyurethane PSAs having special additional properties, such as flame retardancy or electrical conductivity, for example, are described in, for example, EP 1 108 768 A1 (Wong, Tyco) or U.S. Pat. No. 4,855,077 A (Hata et al., Takiron).
Foamed polyurethanes having pressure-sensitive adhesion properties are likewise known. An example that may be mentioned is the publication DE 24 35 217 A (Adsley et al., Adhesive Tapes), and also the descriptions of hydrophilic foams in DE 42 33 289 A (Kenndoff et al., Beiersdorf) and WO 94/07935 A (Kenndoff et al., Beiersdorf).
In principle, as a result of the enlargement of the surface area, foamed polyurethanes have the drawback of a heightened oxidative sensitivity and also a heightened light sensitivity. In practice it has been found that they exhibit strong peel increase on paper and paperboard when bonded for a prolonged period and either cannot be removed without damaging the paper or paperboard or else, particularly in the case of foams made hydrophilic by additions of superabsorbent, interact with the paper or paperboard in such a way that spotting occurs.
Polyurethanes having pressure-sensitive adhesive properties can also be obtained, as demonstrated in JP 2000 073 040 A1 (Toyo) and JP 2000 256 638 A1 (Toyo), by using not only polyethers but also polyesters and also two different catalysts within a polyol component formula. A particular drawback in this case is the increased preparation complexity resulting from the formulas.
JP 2000 328 034 A1 (Toyo), U.S. Pat. No. 3,761,307 A (Dahl) and U.S. Pat. No. 3,925,283 A (Dahl, Continental Tapes) describe pressure-sensitive adhesive polyurethane/ureas which are obtained by incorporating additional amine-type chain extenders or crosslinkers into the polymer. Drawbacks perceived are the complexity in preparation and the presumed excessive peel increase on paper.
DE 21 39 640 A (Dollhausen et al., Bayer) describes a PSA based on an aromatic diisocyanatourethane. A particular drawback is the yellowing tendency, which is typical of aromatic polyurethanes.
In order to achieve pressure-sensitive adhesion properties DE 100 30 908 A1 (Bolte et al., Henkel) and EP 0 081 103 A1 (Miyake et al., Takeda) propose using two different isocyanates within a polyurethane composition. In these cases too the complexity in preparation is perceived to be a drawback.
WO 97/22642 A1 (Chang et al., Bristol Myers Squibb) proposes, for the preparation of a PSA, heating an NCO-terminated prepolymer and a polyhydroxy compound together at a certain temperature until a gel fraction of 30 to 40% is obtained. A disadvantage of this method is the grease strikethrough tendency when the PSA is used to bond paper, this tendency being a result of the relatively low gel content.
U.S. Pat. No. 3,796,678 A (Bartizal, 3M) discloses a polyurethane PSA based on capped isocyanate prepolymers which relies on water or organic solvents for its preparation. The complex nature of the preparation is regarded as a drawback, along with the unavoidable need to use water or solvents.
A polyurethane latex PSA is described in WO 98/31760 A1 (Schrock et al., Dow Chemical). A drawback is the need for drying, which makes it either impossible or at least very time-consuming to obtain blister-free, relatively thick PSA films.
Certain publications define a polyurethane PSA by way of the crosslinking density. GB 1,113,925 A (Weller) and GB 1,216,672 A (Grindley) propose chain lengths of from 130 to 285 chain atoms and, respectively, more than 285 chain atoms between the crosslinking points. In practice it has been shown that controlling the PSA properties by way of the criterion of chain length alone is an impossibility. An insufficient crosslinking density results in the known problem of grease strikethrough in the case of adhesive bonds on paper, while too high a crosslinking density leads to PSAs whose intrinsic adhesiveness is inadequate.
EP 1 088 871 A1 (Heguri et al., Sekisui) prescribes a certain distance between the isocyanate groups, or a certain degree of crosslinking, for the polyisocyanate used. The molecular weight between two isocyanate groups within the polyisocyanate is intended to amount to from 220 to 570. This way of controlling the crosslinking density by way of the chain length within the polyisocyanate is likewise unlikely to mitigate the known problem either that the bond strength is inadequate, particularly for bonds on paper, or otherwise that a tendency is observed for grease spots to be formed.
U.S. Pat. No. 6,040,028 A (Cline et al., Bayer) also defines a polyurethane adhesive (contact adhesive) by way of the molecular weight between crosslinking points. A molecular weight of between 7000 and 16000 is prescribed. Further restrictions are imposed, to the effect, inter alia, that from 0 to 10% of the polyols must have a molecular weight of from 60 to 400 and from 90 to 100% of the polyols must have a molecular weight of from 1800 to 12000.
Here again it must be assumed that the grease strikethrough tendency typical of PSA polyurethanes will occur, since the crosslinking density is relatively low and no indications are given of a possible solution to the problem.
WO 01/62818 A1 (Hansen et al., 3M) proposes reacting isocyanates with two polyols or other materials which react with NCO groups for the purpose of preparing a polyurethane PSA, the components which react with the isocyanates differing from one another in that one has a molecular weight of more than 2000 and one has a molecular weight of less than 2000.
PSAs of this kind are known, inter alia, from the examples in U.S. Pat. No. 5,227,409 A and U.S. Pat. No. 3,437,622 A, which also includes the restriction (according to dependent claim 5) whereby the components which react with the isocyanates are almost exclusively diols. Additionally, polyurethanes with such a composition have already been described in the examples in EP 1 095 993 A1 and EP 1 101 807 A1, the last-mentioned examples not relating to compositions for obtaining PSAs. The proposed reaction product in WO 01/62818 A1 is therefore not definitely a PSA. There are no references to solving the grease strikethrough problem. Nor are there any references to achieving transparency.